1. Field of the Invention
The present invention relates to a driving circuit for a vehicle crew protecting device.
2. Description of the Related Art
An airbag system is well known for a crew protecting device. If a vehicle equipped with an airbag system has a collision, an airbag is inflated by an airbag driving circuit (or an ignition device) to protect a crew, such as disclosed in JP-A-11-70850 or its counterpart U.S. Pat. No. 6,157,246.
The airbag driving circuit includes a mechanical collision detecting sensor, a constant current circuit, a squib, an igniting power transistor, etc. The constant current circuit is a current mirror circuit that is constituted of a current detecting MOS transistor and a current supplying MOS transistor. The source of the current supplying MOS transistor is connected to one end of the squib, and the drain thereof is connected, via the mechanical collision sensor, to a booster circuit for boosting battery voltage. The other end of the squib is connected to the drain of the ignition power transistor, whose source is grounded. The gate of the ignition power transistor is connected to a collision sensing unit.
When a vehicle equipped with such an airbag system has a collision, a wire that connects electric loads and a battery may snap off due to a collision shock. Accordingly, power consumption of the vehicle abruptly reduces, so that the terminal voltage of the output voltage of the booster circuit sharply increases, thereby generating a load dump surge that comes to the airbag driving circuit. Because the igniting power transistor has been turned on, the load dump surge is applied to a portion between the drain and the source of the current supplying MOS transistor, which has been conducting squib igniting current.
That is, the current supplying transistor has to have a capacitor to conduct both the squib igniting current and the load dump surge current.
This amount current increases the loss of the transistor and makes the size of the current supplying transistor become too large for the driving circuit.